The present invention relates to a method of extruding plastics while reducing slippage of the melt at the die wall during the extrusion process so as to avoid sharkskin melt fracture.
Plastics are extruded to produce a variety of products including bottles, fishing line, fiber for clothing, clear plastic wrap, and pipe. Melt fracture is a very important phenomenon in the extrusion of these polymers. Melt fracture places an important limitation on the maximum throughput of an extrusion line. When the rate of extrusion is too high, sharkskin melt fracture develops and creates adverse effects on the product. For example, plastic wrap and bottles will not be transparent and fibers and pipes form unwanted "ribs".
Melt fracture has been attributed to a flow instability due to the slippage of the melt at the die wall. The onset of slip at the wall could be a function of various factors, including surface roughness of the wall and the physio-chemical interactions of the liquid-solid interface between the melt and the wall.
Suppression of melt fracture allows a dramatic increase in the throughput of an extrusion line. The use of additives has increased throughput by a reported factor of three to four. This increase allows a manufacturer to produce the same amount of product using only 25 to 33 percent of the number of extrusion lines needed for production without additives. However, additives do not suppress melt fracture effectively, working only on a hit and miss basis.
Further, it is estimated that the plastics industry spends at least 2 percent of the cost of the resins used in extrusion of plastics on processing additives to suppress the sharkskin melt fracture. Reports from the plastics industry show that in 1993 producers in the United States, Canada and Mexico demanded and used Low Density Polyethylene (LDPE), Linear Low Density Polyethylene (LLDPE), Ethylene Vinyl Acetate (EVA), High Density Polyethylene (HDPE), Polypropylene (PP), Polyvinyl Chloride (PVC), and Polystyrene (PS) at an average price of $0.40/lb. as follows:
______________________________________ DEMAND TOTAL EXTRUDED (Million EXTRUDED RESIN COST RESIN lbs) (Million lbs) (Million $) ______________________________________ LDPE/LLDPE/ 13,708 10,584 4,233.60 EVA HDPE 10,544 2,886 1,154.40 PP 9,147 4,040 1,616.00 PVC 10,605 6,692 2,676.80 PS 5,735 2,581 1,032.40 Total 49,739 26,783 10,713.20 ______________________________________
Based upon these reported figures, the total cost of the additives used by the plastics industry in 1993 to suppress sharkskin melt fracture is about $200 million.